1. Field of the Invention
This invention relates to an ignition circuit for self-purifying creeping discharge spark plugs.
2. Description of the Prior Art
Combustion gas created in internal combustion engines contains carbon composite other than gas containing carbon tar, etc. The carbon composite adheres to and is deposited on the insulator surface of a spark plug causing the insulation resistance of the spark plug to decrease. Such a decrease of the insulation resistance of the spark plug causes a sudden voltage drop in an ignition device having a large output impedance. In this case, if the output voltage of the ignition device becomes lower than the voltage required for producing sparks across the spark plug, no spark discharge occurs which results in a so-called misfire of the spark plug.
The misfire of the spark plug must be avoided to decrease the emission of unburnt exhaust gas from the internal combustion engine. In order to prevent the insulation resistance of the spark plug from decreasing owing to carbon compound adhered to and deposited on the insulator surface of the spark plug, many attempts have been made to design the spark plug such that the front end of the insulator is subject to high temperature so as to burn the carbon composite deposited on the insulator surface, that is, to improve the heat self-purifying property of the spark plug.
In general, the internal combustion engine is operated under such conditions that its speed is changed for a wide range between an idling speed domain and a full speed domain, and as a result, the temperature of the front end of the insulator under the full speed domain becomes significantly higher than that under the idling speed domain. As a result, if the temperature of the front end of the insulator is established such that its temperature reaches a magnitude at which the carbon composite adhered to and deposited on the insulator surface becomes burnt under the idling speed domain, the front end of the insulator becomes excessively heated under the full speed domain, thereby inducing premature ignition. In general, it has been very difficult to design the spark plug on the basis of its heat self-purifying property such that the spark plug can operate without decreasing the insulation resistance of the spark plug due to the carbon composite adhered to and deposited on the insulator surface of the spark plug under the low speed domain without inducing the premature ignition under the full speed domain.
Recently, it has eagerly been requested to provide an ignition system which can avoid a misfire of the spark plug even when the temperature at the front end of the insulator becomes high preventing premature ignition and which can purify the exhaust gas from the internal combustion engine without deteriorating its performance.
It might be possible to provide an ignition system which can operate for a wide temperature range if the self-purifying property of the spark plug is electrically attained without recourse to the conventional spark plug which makes use of its heat self-purifying property. Attempts have been made to provide a capacitor discharge ignition device for electrically removing the carbon composite adhered to and deposited on the discharge path of a creeping discharge spark plug since the creeping discharge spark plug has been developed.
In such conventional capacitor discharge ignition device, use is made of an oscillation step-up circuit including, for example, a transistor type DC-DC converter so as to increase a battery voltage up to several hundred volts. In this case, the battery functions to charge a capacitor. Provision is also made of a silicon controlled rectifier having a gate supplied with a pulse signal so as to discharge the electric charge of the capacitor through a primary winding of an ignition coil. A secondary voltage induced in a secondary winding of the ignition coil is rapidly stepped up, and as a result, the discharge voltage at the secondary winding is kept substantially constant irrespective of a leakage resistance due to contamination of the spark plug, etc.
It has also been proposed to use a combination of an electric circuit comprising a capacitor and a resistor and the conventional spark plug including a center electrode and a ground electrode opposed to the center electrode and forming a spark gap therebetween through which pass sparks. In this case, the capacitor is connected in parallel with the spark gap and the resistor is connected in series with the capacitor. The use of such combination of the RC circuit and the conventional spark plug has the following drawbacks.
(1) When the engine operates at a low speed, carbon composite adhered to and deposited on the insulator can be removed by the RC circuit. But, the step of removing the carbon composite is effected independently of the spark discharging or firing step, so that considerable energy is consumed in the carbon composite removing step, thereby reducing the energy required for firing the air-fuel mixture gas in the combustion chamber.
(2) When the engine operates at a high speed, the RC circuit produces a large discharge current that tends to deteriorate the electrodes.
An outboard engine is used under two extremely different conditions, i.e. used at a low speed in the case of, for example, idling, trawling, etc. and used also at a high speed for a long time in the case of fully opening throttle valves, and must satisfy those two conditions and hence must be provided with a wide temperature range spark plug. Such a wide temperature range spark plug having an excellent reliability and durability and particularly adapted for outboard engines is now urgently in demand.